For parameters that are required as part of an API, prefix them
by "_" to make it clear they are unused

This pattern is available from python3.5 and provides a simpler
understanding of what is going on

This will allow cross-junction dependencies to be listed as strings on a
single line.

As part of this, some logic around initializing `Dependency()` objects
have been moved out of `_extract_depends_from_node()` method into the
constructor of `Dependency()` class, to keep all related code in one
place.

* _loader/types.py: While initializing `Dependency` objects, attempt to
                    split filenames, only if no `junction` was specified
                    explicitly. If a `junction` was specified, then
                    filenames with `:` in their names will result in an
                    error.

* _loader/loadelement.py: Refactor logic to initialize `Dependency()`
                          objects to move it to the `Dependency()`
                          constructor.

* tests/frontend/buildcheckout.py: Add tests to ensure the above.

* _versions.py: Bump BST_FORMAT_VERSION.

Fixes #809.

This test ensures the overlap failure vs warning policy in one
project only ever affects the artifacts created for the project
which declares it and does not force it's policy onto another
consuming project.

A regression test against issue #926

Will check and move old artifact directory if it exists, and create
symlink linking old directory to new.

Part of #870

Makes artifactdir and builddir obsolete.

Fixes #870

A non-integration test works just as well for this and is a lot quicker,
especially on WSL.

This allows use of variables such as %{prefix} and matches the
documentation. This is a breaking change.

Fixes #721